The present invention relates generally to gas turbine engines, and, more specifically, to air bleeding therein.
Modern aircraft are typically powered by turbofan aircraft engines. The turbofan engine is a specialized form of gas turbine engine in which air flows through a fan and compressor that pressurizes the air in turn which is then mixed with fuel in a combustor for generating hot combustion gases.
A core engine includes the compressor and combustor and cooperating high and low pressure turbines that extract energy from the combustion gases for powering the compressor and fan, respectively.
Most of the inlet air is pressurized by the fan and bypasses the core engine for producing a majority of propulsion thrust for powering the aircraft in flight. A portion of the fan air is directed into the core engine wherein it is further pressurized in multiple stages of increasing pressure in the compressor therein.
The turbofan engine not only powers the aircraft in flight but also provides customer bleed air thereto for typical use by the aircraft manufacturer and operator in the environmental control inside the aircraft or for deicing the aircraft wings in two examples.
The typical turbofan engine includes a multistage axial compressor having various bleed circuits initiating therein for extracting pressurized air at different pressure and temperature as required for various purposes.
In one exemplary commercial turbofan aircraft engine on sale in the United States for more than one year, pressurized air is bled from the compressor and channeled through a precooler or heat exchanger in a primary circuit thereof. A secondary circuit bleeds pressurized air from the fan bypass duct to the same heat exchanger. The fan air is used in the heat exchanger for precooling the hot bleed air provided by the compressor. The fan air is then dumped overboard from the heat exchanger, and the compressor air is channeled to the aircraft for further use therein.
The heat exchanger is used for reducing the temperature of the hot bleed air from the compressor below the auto-ignition temperature of the fuel stored in the aircraft wings for providing a corresponding safety margin.
Cooling compressor bleed air in this fashion commonly occurs in different types of aircraft that share the common use of the air-to-air heat exchanger for the different compressor and fan air circuits. The fan air circuit necessarily requires a suitable inlet disposed inside the bypass duct downstream of the fan, and must be suitably designed for maximizing aerodynamic performance thereof.
The modern turbofan aircraft engine enjoys substantial efficiency of performance, and is designed for high durability and life. Accordingly, the fan air bleed duct is precisely designed in configuration and flow area for maximizing pressure recovery of the speeding bypass fan air and thereby maximize engine performance.
The bleed duct may have many configurations, and in the commercial application disclosed above the bleed duct is mounted in the fan bypass duct for receiving the fan air in substantially direct alignment along the longitudinal axis of the bleed duct.
The bleed duct includes a valve between its outlet and the inlet to the heat exchanger for controlling bleed flow therethrough. When the valve is closed at the outlet end of the bleed duct, the inlet end of the bleed duct remains open and is subject to the rush of incoming fan air. This configuration may result in the formation of a Hartmann Generator that causes unstable dynamic pressure oscillations inside the closed bleed duct which can lead to sonic fatigue and subsequent damage of the associated parts.
The typical solution to this problem is allowing the bleed valve to remain partially open when it would otherwise be closed during the aircraft operating cycle to prevent dynamic instability in the bleed duct and avoid damage thereto.
However, in the continuing development of the commercial application described above, it is undesirable to leave the bleed valve even slightly open when it should be closed for increasing the overall efficiency of the turbofan engine.
This presents a significant design problem since the basic configuration of the turbofan engine has been fixed based on previous development and expense rendering impractical the redesign thereof.
There are many components found in the bleed system for the compressor and fan, including the cooperating heat exchanger therefor, for which a change in inlet design of the bleed duct could have adverse consequences requiring further development, time, and cost.
Accordingly, it is desired to provide an improved bleed duct having minimal changes for permitting operation thereof when the valve is closed without undesirable dynamic pressure oscillations, and without degrading performance of the bleed duct when the valve is open.